There is a very wide installed base of dimmer circuits in mains lighting arrangements. The vast majority of dimmer circuits use phase-cut dimmers, in which power is supplied to the lighting unit during only one part of the mains phase; the power being cut during the remainder. The cut part may either be the trailing part of the phase: such trailing-edge phase cut dimmers normally have a transistor to cut the phase; alternatively the cut part may be the leading part of the phase; such leading-edge phase cut dimmers are more common and normally use a triac to cut the phase.
In order to operate properly, the triac requires a certain level of current (so-called sync or synchronisation current) through it during the first (cut) part of the phase, in order to determine when to switch on, at a predetermined switch-on voltage; also, and more significantly, the triac requires a (generally higher) latching current through it whilst it switches and settles to an on-state. In order to ensure that it remains on for the remainder of the phase, a third current called a holding current is required.
This is illustrated in FIG. 1 which shows two part-rectified mains half-cycles 110 (shown partially dotted), and the voltage shape 120 of the voltage supplied to the load. The voltage shape 120 includes an initially off period 122, and a rising leading-edge 124. The lower part of the figure shows the current 130 through the device which is required to properly operate the triac. This includes a synchronization current 132 during the initial (cut) part of the phase; a latching current 134 around the leading-edge; and a hold current 136 throughout remainder of the mains half cycle.
Conventional incandescent lighting units generally have a low impedance, and thus the current through them is sufficiently high to meet the current requirements 130 of the dimmer. However, modern forms of lighting such a compact fluorescent (CFL) and light-emitting diode (LED) are generally more efficient, and in particular offer a high impedance, such that for lighting sources such as LEDs in particular, it is no longer the case that the lighting unit will sink sufficient to ensure that the triac properly operates. High impedance lighting sources are thus generally not compatible with conventional triac-based dimming units without modification.
It is known to provide a so-called bleeder circuit in parallel with the lighting unit in order to sink current and ensure the triac operates correctly. Such bleeder circuits are known, for instance NXP™ SSL 210x series of LED lighting controllers. Since such a bleeder circuit does not directly contribute to the luminous output of the LEDs lighting, it represents a source of loss; it is known to disable the bleeder during part of the mains phase when it is not required.
It will be appreciated that, although mains lighting is one example of an application which can be used with a phase cut dimmer, the invention is not limited thereto, but extends to other applications operable with a leading-edge phase cut dimmer, such as, for instance cooling fans.